In recent years, a system for processing information using a plurality of processors which operate in parallel has been popular. For example, a blade server is an information processing system which includes a plurality of blades. Each of the blades includes a CPU and a memory, and operates as a computer. The plurality of blades are accommodated in a chassis in the specified shape.
Each of the blades can transmit and receive data to/from other blades. Namely, the blades are connected through a transmission link. According to one embodiment, the blades are connected through a metal cable transmitting an electrical signal, such as a coaxial cable. In this case, a signal conforming to, e.g., a PCI (Peripheral Components Interconnect) is transmitted between the blades.
However, further increases in speed of information processing have been requested, and further increases in speed of a signal transmitted between blades have also been needed. For example, the transmission between blades of a high-speed signal with a speed exceeding 10 Gb/s is sometimes requested. For that reason, a configuration in which blades are connected through an optical interface, instead of the electric interface as described above, has been developed.
When an optical interconnection in which blades are connected through an optical interface is realized, each of the blades includes an optical module for transmitting and receiving an optical signal. The optical module includes an optical transmitter and an optical receiver. In a blade server which includes a large number of blades, for example, each of the blades may include a large number of optical transmitters and optical receivers in order to transmit and receive data to/from a large number of other blades. In this case, in each of the optical modules, the large number of optical transmitters and optical receivers are preferably implemented with a high density.
As a related art, an optical waveguide board described below has been proposed. This optical waveguide board includes a base material, a film, an optical element, and an optical path switch. The film, in which an optical waveguide is formed that includes a core that is an optical path propagating an optical signal and a clad surrounding the core, is provided on a principal surface of the base material. The optical element is implemented on at least either of the base material and the film, and is optically coupled to the optical waveguide. The optical path switch changes the optical path propagating an optical signal in a desired direction. (For example, Japanese Laid-open Patent Publication No. 2004-258065)
As another related art, an optical module which includes an optical element which is flip-chip implemented on a board, an optical waveguide which is formed on the board and optically coupled to the optical element, and an underfill resin which is filled in between the board and the optical element and covers an optical connecting point between the optical element and the optical waveguide, has been proposed. (For example, U.S. Pat. No. 6,661,939)
As still another related art, an optical module which includes a flexible printed circuit on which optical elements (E/O and O/E) are flip-chip mounted, and an optical waveguide which is provided on the flexible printed circuit, has been proposed. A 45-degree mirror is formed in the optical waveguide so as to be optically coupled to a light emitting surface of the E/O and a photo detector surface of the O/E. (For example, Cost-effective On-board Optical Interconnection using Waveguide Sheet with Flexible Printed Circuit Optical Engine, Takashi Shiraishi, et al., OFC/NFOEC 2011, OTuQ5)
In the known arts, it is not easy to implement optical transmitters and optical receivers in an optical module with a high density. For that reason, when the numbers of optical transmitters and optical receivers implemented in an optical module increase, the size of the optical module increases. In other words, when the size of the optical module is determined according to a standard, etc., it is difficult to increase the numbers of optical transmitters and optical receivers implemented in the optical module.
Further, the optical module includes optical devices and electric devices. When an electric circuit is a differential circuit, a specified fan-out is required. Additionally, when transmission signal lines (signal lines for propagating signals to E/O converters) and received signal lines (signal lines for propagating output signals from O/E converters) are not appropriately designed on a board of the optical module, the size of the board increases. Here, if the pitch or spacing of these signal lines are made smaller in order to reduce the size of the board, a crosstalk between channels and a crosstalk between a transmission signal and a received signal increase. Accordingly, it is required to widen the pitch or spacing of the signal lines in order to suppress the crosstalks, and therefore the board of the optical module becomes larger.
When the optical module as described above is provided on a blade of a blade server, it is preferable that an area occupied by the optical module in the blade is small. In order to reduce the area for mounting the optical module, an optical module board on which one or more optical modules are mounted may be vertically attached to a mother board of the blade.
However, as described above, it is difficult to reduce the size of the optical module itself. For that reason, if the optical module board on which one or more optical modules are mounted is vertically attached to the mother board of the blade, the width of the blade increases. Accordingly, in the configuration where the optical module board on which one or more optical modules are mounted is vertically attached to the mother board of the blade, it is requested to increase in mounting density of the optical modules and reduce the size (in particular, reduce the height) of the optical module.